This study is a prospectively and retrospectively enrolled, post-market, open-label, non-randomized, single institution evaluation of clinical outcomes of primary or revision knee surgery patients who meet the eligibility criteria and received devices from the Triathlon Hinge Knee System according to its indications for use (IFU). Clinical evaluation for all cases will include data collection of patient demographics, surgical details, early postoperative status, functional and clinical outcomes, and survivorship at 1, 2, 6, and 10-years.